1. Field of the invention
The invention relates to an apparatus for fabricating single-crystal silicon, and more particularly to an apparatus for fabricating single-crystal silicon based on the Czochralski method.
2. Description of the Prior Art
High pure single-crystal silicon is mainly used as substrates in semiconductor fabricating. Generally, a method for fabricating single-crystal silicon is based on the Czochralski (hereinafter referred to as "CZ"). As shown in FIG. 8, lumpy poly-silicon materials are fed into a quartz crucible 4 disposed inside the main chamber 1 of an apparatus for fabricating single-crystal silicon. The lumpy poly-silicon materials are melted into a melt 5 by a loop heater 6 surrounding a graphite crucible 3 in which the quartz crucible 4 is disposed. Furthermore, disposing a seed on a seed holder 21, the seed holder 21 and graphite crucible 3 rotate in the same direction or in different directions. Meanwhile the seed holder 21 is pulled up to grow single-crystal silicon 12 on the seed.
A thermal shield member 22 made of graphite and closely extending to the melt 5 is located on the top of the main chamber 1. The above-mentioned thermal shield member 22 is used to control the flow rate of an inert gas led into from the top of the main chamber 1 and to block radiant heat from the melt 5 and heater 6 in order to cool and provide heat conservation through the whole temperature region along single-crystal silicon 12, thereby easing crystallization and raising the yield of single-crystal silicon.
Referring to FIG. 9, an apparatus for fabricating single-crystal silicon has a thermal shield member 23 of a hollow, inverted truncated conical shape with openings at both ends. The above-mentioned thermal shield member 23 installed on the top of a heat conservation member 7 by means of a support 24 leads an inert gas into the surroundings of single-crystal silicon 12 to form a current flowing from the center of the quartz crucible 4 to the bottom of the main chamber 1 via internal edges, thereby exhausting silicon dioxide (SiO.sub.2) created by a melt 5 and metal vapor created by a graphite crucible 3, blocking a single crystallization gas in order to raise crystallization rate without dislocation, and increasing the temperature gradient in diameter and axis directions of single-crystal silicon on the solid-liquid interface so as to enhance pulling stability.
The radiation heat which is emitted from hot zone parts including a main heater to the pulling single-crystal silicon is blocked by a thermal shield member as shown in FIG. 8 or FIG. 9. Since the temperature gradient in diameter and axis directions on the solid-liquid interface is increased, crystallization is easy, thereby enhancing the pulling speed for single-crystal silicon and raising productivity. However, by using the thermal shield member to make the whole temperature region in the axis direction of the single-crystal silicon extremely cool resulting in a temperature gradient of 20.degree. C..about.30.degree. C./cm causes two problems as follows:
(1) When the pulling single-crystal silicon passes the temperature region of 1000.degree. C..about.1200.degree. C., since the above-mentioned temperature region is not cooled slowly, as-grown defect density can not be decreased and a low breakdown voltage of an oxide film results.
(2) Since the thermal shield member 22 is fixed on the top of the main chamber 1 and is closely extended to the melt 5, if lumpy poly-silicon materials are fed into the quartz crucible 4, interference between lumpy poly-silicon materials and the bottom of the thermal shield member 22 occurs. Hence, the lowering of quartz crucible 4, which is necessary during the melting lumpy poly-silicon materials requires extra time. Referring to FIG. 9, the thermal shield member 23 having a lift can be raised to the top of the main chamber 1, thus preventing the interference between lumpy poly-silicon materials and the bottom of the thermal shield member 23.